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Find a point on the curve y = 3x2 – 9x + 8 at which the tangents are equally inclined with the axes. |
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Answer» Given: The curve is y = 3x2 – 9x + 8 Differentiating the above w.r.t x ⇒ y = 3x2 – 9x + 8 ⇒\(\frac{dy}{dx}\) = 2 x 3x2 – 1 – 9 + 0 ⇒ \(\frac{dy}{dx}\) = 6x – 9 ...(1) Since, the tangent are equally inclined with axes i.e, θ = \(\frac{\pi}{4}\) or θ \(=\frac{-\pi}{4}\) \(\therefore\) \(\frac{dy}{dx}\) = The Slope of the tangent = tanθ ⇒ \(\frac{dy}{dx}\) = tan( \(\frac{\pi}{4}\) ) or tan( \(\frac{-\pi}{4}\) ) ⇒ \(\frac{dy}{dx}\) = 1or – 1 ...(2) \(\therefore\) tan( \(\frac{\pi}{4}\) ) = 1 From (1) & (2),we get, ⇒ 6x – 9 = 1 0r 6x – 9 = – 1 ⇒ 6x = 10 0r 6x = 8 ⇒ x = \(\frac{10}{6}\) or x = \(\frac{8}{6}\) ⇒ x = \(\frac{5}{3}\) or x = \(\frac{4}{3}\) Substituting x = \(\frac{5}{3}\) or x = \(\frac{4}{3}\) in y = 3x2 – 9x + 8,we get, When x = \(\frac{5}{3}\) ⇒ y = 3( \(\frac{5}{3}\) )2 – 9( \(\frac{5}{3}\) ) + 8 ⇒ y = 3( \(\frac{25}{9}\) ) – ( \(\frac{45}{3}\) ) + 8 ⇒ y = ( \(\frac{75}{9}\) ) – ( \(\frac{45}{3}\) ) + 8 taking LCM = 9 ⇒ y = ( \(\frac{(75\times1)-(45\times3)+(8\times9)}{9}\) ) ⇒ y = ( \(\frac{75-135+72}{9}\) ) ⇒ y = ( \(\frac{12}{9}\) ) ⇒ y = ( \(\frac{4}{3}\) ) ⇒ y = 3( \(\frac{4}{3}\) )2 – 9( \(\frac{4}{3}\) ) + 8 ⇒ y = 3( \(\frac{16}{9}\) ) – ( \(\frac{26}{3}\) ) + 8 ⇒ y = ( \(\frac{48}{9}\) ) – ( \(\frac{36}{3}\) ) + 8 taking LCM = 9 ⇒ y = ( \(\frac{(48\times1)-(36\times3)+(8\times9))}{9}\) ) ⇒ y = ( \(\frac{48-108+72}{9}\) ) ⇒ y = ( \(\frac{12}{9}\) ) ⇒ y = ( \(\frac{4}{3}\) ) Thus, the required point is ( \(\frac{5}{3}\), \(\frac{4}{3}\)) & ( \(\frac{4}{3}\), \(\frac{4}{3}\)) |
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